1. Field of the Invention
The present invention relates to a radar device using a transmitted wave that is a frequency modulated continuous wave (FM-CW) and, in particular, it relates to a radar device that receives and scans reflection waves resulted from the transmitted electric wave by means of Digital Multi-beam Forming (DBF).
2. Description of the Related Art
It is well known that various radar devices for performing reception and scanning using DBF have been developed. Typically, the radar device of this type is basically provided with one transmitting antenna and a plurality of receiving antenna, wherein an electric wave is transmitted from the transmitting antenna and reflection waves resulted from the transmitted electric wave are received by the plurality of antennas.
However, the radar device of such configuration needs receivers, the number of which is equal to the number of the receiving antennas and, in order to improve scanning accuracy, many receivers must be provided. Therefore, there is a problem in that, as the number of the receivers is increased, the weight and size of the radar device is increased and a great deal of electric power is required.
Therefore, in order to solve this problem, radar devices intended for size and weight reduction are disclosed in, for example, Japanese examined patent publication (Kokoku) H06-68542, Japanese unexamined patent publication (Kokai) H11-311668, Japanese unexamined patent publication (Kokai) H11-160423 and so on. These radar devices are configured so that a plurality of antennas are connected to one receiver via a switch. Alternatively, a plurality of antennas are divided into several groups or, more specifically, for example, a receiving antenna array consisting of multiple receiving antennas is divided into receiving antenna groups, each of which includes four receiving antennas, and receivers are provided by one for every four receiving antennas so that the antennas in each group are connected to each corresponding receiver. In such configuration, when reflected waves of the transmitted electric wave are received, the plurality of receiving antennas are switched sequentially and connected to the receivers. This allows radar signals to be received by each antenna in a time division manner.
Such configuration can reduce the number of receivers corresponding to the plurality of antennas to one or a number that is less than the number of the plurality of antennas, which will result in reduction of the size and cost of the device.
In this connection, the electric wave used by the radar device is, for example, a high-frequency electric wave such as one in the 76 GHz band. This means that the signal that is processed in the transmission path from the receiving antennas to the receiver also has such a frequency. Typically, switches that can switch such high-frequency signals have only two or three inputs.
Therefore, when four or more antennas must be switched, a plurality of switches are used. For example, multiple switching can be implemented by combining unit switches having one input and two switched outputs (SPDT) or having one input and three switched outputs (SP3T) in a multistage manner. Here, the unitary switches may be planar circuit type high-frequency switches such as MMIC (microwave monolithic integrated circuits) or HIC (hybrid integrated circuits).
However, when the switches are connected in a multistage manner, there is a problem in that the signal is attenuated increasingly every time the signal passes each switch and, therefore, as the number of the receivers is reduced, the number of the switch stages is increased and the receiving sensitivity is degraded.
As a result, a radar device that has a relatively simple configuration and that can prevent the receiving sensitivity from being degraded is disclosed in, for example, Japanese unexamined patent publication (Kokai) 2000-155171. In the radar devices described above, one transmitting antenna corresponding to a plurality of receiving antennas is provided. In contrast, in the radar device shown in Japanese unexamined patent publication (Kokai) 2000-155171, a plurality of transmitting antennas is used in a switching manner so that the number of the receiving antennas and, thus, the number of the switches to switch the receiving antennas can be reduced. This configuration improves the receiving sensitivity and reduces the number of the antennas and switches, along with the cost of the device.
In this radar device, three transmitting antennas and two receiving antennas are connected to switching means and, in turn, a transmitter having an oscillator for outputting a high-frequency signal and a receiver are connected to the switching means. The receiver is synchronized with an oscillating signal from the oscillator and transmits received signals from the receiving antennas to a signal processing controller. Then, the signal processing controller performs signal processing for Digital Multi-beam Forming (DBF) based on the received signals and controls switching of the transmitting and receiving antennas in the switching means.
These transmitting and receiving antennas are aligned on an identical plane and in an identical straight line and the number of the antennas is less than that in the conventional radar devices. By configuring the radar device as described above, the radar device can be manufactured easily and with a reduced cost and, further, in applications such as a car-mounted radar device and the like, an entire radar device can be shaped so that it is suitable for being mounted on a vehicle.
In the radar device shown in Japanese unexamined patent publication (Kokai) 2000-155171 above, when the distance between the two receiving antennas is L, the three transmitting antennas are disposed so that each distance between the adjacent two transmitting antennas is 2 L.
In this connection, in other conventional radar devices, in order to equip the radar device with six receiving antennas and one corresponding receiver, the switch must have been in a two-stage configuration that is comprised of two switches having one input and three switched outputs and another one switch having one input and two switched outputs. In contrast, in the radar device shown in Japanese unexamined patent publication (Kokai) 2000-155171 above, a switch must be provided also at the transmitting side but one-stage switch suffices to switch between two receiving antennas.
Further, in this radar device, if it is desired to further increase the number of channels so as to reduce the directivity of the obtained beams, a receiving antenna having antenna characteristics identical to those of other receiving antennas is disposed additionally with a distance L. Then, the three transmitting antennas are disposed so that each distance between the adjacent two transmitting antennas is 3 L. In the radar device with such antenna configuration, if six antennas are used (i.e. three transmitting antennas, and two receiving antennas plus one added receiving antenna), three channels can be added and, thus, beams of nine channels can be obtained.
As described above, according to this radar device, the attenuation of the received signal in the switches can be reduced and, by using the antennas the number of which is less than that in the conventional radar device, a number of channels that is more than that of the antennas can be implemented.
Further, according to this radar device, the number of channels that is more than that of antennas can be implemented and beams with narrower directivity can be obtained. For example, in the case of nine channels, ten antennas, that have been needed in the conventional radar device, can be reduced to six. However, when the antennas are assembled in the radar device, the antennas are arranged side by side in a line. Because these six antennas are arranged so that each distance between the transmitting antennas is 2 L, a space for ten antennas is needed.
On the other hand, for example, when such a radar device is mounted on a car as one of various types of car-mounted electronic equipment, in order to assure that the electric wave is radiated toward the front of the car, the radar device must be mounted in a limited and narrow space. The radar device used in such an environment must be made as small as possible. Thus, even though the number of antennas could be reduced in the radar device described above, it would be insufficient in terms of size. Moreover, in view of recognition of objects in front of the car and safety of the car driving, there is a need for a radar device having higher performance and lower cost.
Thus, it is an object of the present invention is to provide a radar device that reduces the number of antennas for receiving and scanning reflected waves resulted from a transmitted electric wave by means of Digital Multi-beam Forming (DBF) and that can implement multiple channels with smaller size and higher performance as well as lower cost.